1. Field of the Invention
The present invention relates to nano-structures provided with narrow pores, which can be used in various fields, for example, as functional materials and structural materials for electronic devices, optical devices, micro devices, and the like.
2. Description of the Related Art
With respect to thin films, narrow wires, and small dots made of metals and semiconductors, unique electrical, optical, and chemical properties may be demonstrated when movement of electrons is confined to a size less than a specific length. In view of this, there has been a growing interest in materials having fine structures with sizes of less than several hundreds of nanometers (nm) (i.e., nano-structures) as functional materials.
Nano-structures are produced, for example, by semiconductor processing techniques, such as micro pattern writing techniques including photolithography, electron-beam lithography, X-ray lithography, and the like.
In addition to the production methods described above, attempts have been made to produce new nano-structures based on naturally formed regular structures, namely, structures formed in a self-ordering manner. Since there is a possibility of producing finer, more special structures in comparison with those produced by conventional methods, much research has been conducted.
One self-ordering method is anodization in which nano-structures having nano-size narrow pores can be formed easily and controllably. For example, anodized alumina is known, which is produced by anodizing aluminum or an alloy thereof in an acidic bath.
When an Al plate is anodized in an acidic electrolytic bath, a porous oxide film is formed (for example, refer to R. C. Furneaux, W. R. Rigby, and A. P. Davidson, NATURE, Vol. 337, p.147 (1989)). As shown in FIG. 10, the porous oxide film is characterized by a geometric structure in which extremely fine cylindrical narrow pores (nano-holes) 14 having diameters of several nanometers to several hundred nanometers are arrayed in parallel within distances of several nanometers to several hundred nanometers. The cylindrical narrow pores 14 have high aspect ratios and highly uniform cross-sectional diameters.
It is also possible to control the structure of the film to a certain extent by the selection of the anodizing conditions. For example, it is possible to control, to a certain extent, the distance between narrow pores by the anodizing voltage, the depth of the pores by time, and the pore diameter by a pore-widening treatment.
Furthermore, as an example of controlling the array of narrow pores, it has been reported by Masuda et al. that ordered nano-holes having a honeycomb array are formed by anodizing under suitable anodizing conditions (Masuda, Kotaibutsuri (Solid State Physics) 31, 493 (1996)).
Another example has been reported by Masuda et al., in which an Al film sandwiched between insulators is anodized in the film surface direction with the aim of arraying narrow pores in a matrix (Appl. Phys. Lett. 63, p.3155 (1993)).
Various applications have been attempted in view of the peculiar geometric structure of anodized alumina as described above. As described in detail by Masuda, for example, anodized films are used as coatings by taking advantage of their wear resistance and dielectric properties, and detached films are used as filters. Moreover, by using techniques for filling a metal or a semiconductor into nano-holes and replication techniques of nano-holes, application to various fields has been attempted, such as coloring, magnetic recording media, electroluminescent devices, electrochromic devices, optical devices, solar cells, and gas sensors. Application to a number of other fields is also expected, for example, to quantum well devices such as quantum wires and MIM devices, and molecular sensors which use nano-holes as chemical reaction fields (Masuda, Kotaibutsuri (Solid State Physics) 31, 493 (1996)).
It is an object of the present invention to provide a nano-structure in which the structure is controlled in a more sophisticated manner.
That is, it is an object of the present invention to control the arrays, distances, positions, directions, etc. of narrow pores in structures having narrow pores formed by anodizing.
It is another object of the present invention to provide novel nanometer-scale structures and devices by controlling the arrays, distances, positions, directions, etc. of narrow pores.
The objects described above are achieved by the following production methods in accordance with the present invention.
In one aspect, a method of producing a structure having narrow pores, in accordance with the present invention, includes a first step of bringing pore-guiding members into contact with upper and lower surfaces of a member comprising aluminum as a principal ingredient, and a second step of anodizing the member comprising aluminum as the principal ingredient to form narrow pores. The pore-guiding members contain the same material as a principal ingredient.
In another aspect, a method of producing a structure having narrow pores, in accordance with the present invention, includes a first step of disposing a pore-guiding member and a member comprising aluminum as a principal ingredient having a predetermined pattern on a substrate, the pore-guiding member being in contact with the periphery of the pattern of the member comprising aluminum as the principal ingredient, and a second step of anodizing the member comprising aluminum as the principal ingredient to form narrow pores.
In another aspect, a method of producing a structure having narrow pores, in accordance with the present invention, includes a first step of covering the periphery of a rod-like member comprising aluminum as a principal ingredient with a pore-guiding member, and a second step of anodizing the member comprising aluminum as the principal ingredient to form narrow pores.
In another aspect, a method of producing a structure having narrow pores includes a first step of covering the periphery of a rod-like first pore-guiding member with a member comprising aluminum as a principal ingredient and further covering the member comprising aluminum as the principal ingredient with a second pore-guiding member, and a second step of anodizing the member comprising aluminum as the principal ingredient to form narrow pores.
In another aspect, a method of producing a structure having narrow pores, in accordance with the present invention, includes a first step of bringing a first pore-guiding member and a second pore-guiding member into contact with upper and lower surfaces of a member comprising aluminum as a principal ingredient, and a second step of anodizing the member comprising aluminum as the principal ingredient to form narrow pores. At least one of the first pore-guiding member and the second pore-guiding member is electrically conductive.
As described above, in the first aspect of the present invention, xe2x80x9cthe pore-guiding members contain the same material as a principal ingredientxe2x80x9d, which means that, if each pore-guiding member contains an element such as a metal as a principal ingredient, the pore-guiding members contain the same element, or if each pore-guiding member contains a compound as a principal ingredient, the pore-guiding members contain the same compound. Basically, it is acceptable in the present invention if the pore-guiding members have the same chemical properties (such as stability to a solution used in anodization) and the same electrical properties (such as an electric field generated during anodization).
Additionally, xe2x80x9ca principal ingredientxe2x80x9d in the present invention refers to an ingredient having the highest content among elements and/or compounds contained in a given member.
In accordance with the methods of the present invention, narrow pores of anodized alumina can be formed in the direction parallel to the interface between the pore-guiding member and aluminum (resultant anodized alumina). Furthermore, by appropriately bringing the pore-guiding member into contact with the periphery of the aluminum film having a predetermined pattern on the substrate, the anodized alumina having narrow pores in which the direction is controlled in parallel to the interface between the pore-guiding member and aluminum can be formed by patterning.
In the present invention, by using an electrically conductive material as the pore-guiding member, in the initial stage of forming narrow pores, control of the structure can be increased, and a porous body having excellent uniformity in the shape (narrow-pore diameters, etc.) from the outermost surface to the bottom can be produced.
Furthermore, by appropriately selecting the thickness of the pore-guiding member, the thickness of the member comprising aluminum as the principal ingredient, anodizing voltages, etc., the pore array pitch, the pore diameter, etc. may be controlled.
Furthermore, by disposing a pore-terminating member on the member comprising aluminum as the principal ingredient, narrow pores may be formed highly uniformly at a predetermined length.
That is, in accordance with the methods of the present invention, the position, length, pitch, direction, pattern, etc. of narrow pores having nanometer size diameters can be controlled.
Furthermore, with respect to structures which are produced by embedding a functional material, such as a metal or a semiconductor, into the narrow pores formed by the methods described above, there are possibilities of application to new electronic devices.
The present invention enables anodized alumina to be used for various fields, such as quantum wires, MIM devices, molecular sensors, coloring, magnetic recording media, electroluminescent devices, electrochromic devices, optical devices such as photonic bands, electron emitters, solar cells, gas sensors, coatings having wear resistance and dielectric properties, and filters, and the present invention contributes to the significant expansion of applications for anodized alumina.